Robotics 10

NPN vs PNP

A transistor has three legs:
a collector, an emitter and a base. Below is the symbols for an NPN and a
PNP transistor.

NPN
transistor

PNP
transistor

Transistors as
switches

The easiest way to understand
transistors is to think of them as switches. You can switch a big current
(between the collector and emitter) with a much smaller current (in the
base). Lets look at an example:

NPN
transistor as a switch (on)

NPN
transistor as a switch (off)

Transistors is also handy to convert between different voltages (5V
and 12V in the example above.)

While an NPN transistor
conducts when a current flows into the base, a PNP transistor will conduct
when no current flows into the base and stop conducting when a current
flows into the base.

PNP
transistor as a switch (on)

PNP
transistor as a switch (off)

Transistors

Function

Transistors amplify current, for example they can be used to amplify the small output
current from a logic chip so that it can operate a lamp, relay or other high
current device. In many circuits a resistor is used to convert the changing
current to a changing voltage, so the transistor is being used to amplify
voltage.

A transistor may be used as a switch (either fully on with maximum
current, or fully off with no current) and as an amplifier (always partly
on).

The amount of current amplification is called the current gain, symbol
hFE.

Types of transistor

Transistor circuit
symbols

There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to
the layers of semiconductor material used to make the transistor. Most
transistors used today are NPN because this is the easiest type to make from
silicon.

The leads are labelled base (B), collector (C) and emitter (E).These terms refer to the internal operation of a
transistor but they are not much help in understanding how a transistor is used,
so just treat them as labels!

Transistor currents

The diagram shows the two current paths through a transistor. You can
build this circuit with two standard 5mm red LEDs and any general purpose low
power NPN transistor (BC108, BC182 or BC548 for example).

The small base current controls the larger collector current.

When the switch is closed a small current flows into the base (B) of
the transistor. It is just enough to make LED B glow dimly. The transistor
amplifies this small current to allow a larger current to flow through from its
collector (C) to its emitter (E). This collector current is large enough to make
LED C light brightly.

When the switch is open no base current flows, so the transistor
switches off the collector current. Both LEDs are off.

A transistor amplifies current and can be used as a
switch.

This arrangement where the emitter (E) is in the controlling circuit
(base current) and in the controlled circuit (collector current) is called common emitter mode. It is the most widely used arrangement for
transistors so it is the one to learn first.

The last bit is what confuses a number of people. Let's say you have a 9V battery connected to a 100 ohm resistor and then connected to and NPN transistor and finally to ground. Let's further assume that the beta (Hfe) of the transistor is 250. This is shown below in figure 3.

Figure 3) A simple circuit with a 9V battery and a resistor

What is the maximum current that can flow through the transistor? If we replace the transistor with a piece of wire, then using Ohm's law we can calculate the current flow to be:
I = V/R

I = 9 volts/100 ohms

I = 9/100 or .09 Amps (90 mA).

Now when the current is present, if it has a current of 2mA flowing into the base the maximum current the transistor will conduct is 2mA * Hfe, substituting in 250 for Hfe gives us 2 mA * 250 for a total of 500 mA.

And yet we know that if the transistor was a wire the most we would see is 90mA. So the actual answer is this "The amount of current that flows through the transistor is the lesser of the available current (90mA) and Ibe*Hfe. 500mA "

Generally if you apply enough base current that the maximum current possible will always flow, then the transistor is operating like a switch (rather than operating like an amplifier). In this mode, the transistor issaid to be saturated.

Transistor leads for some common case
styles.

Connecting

Transistors have three leads which must be connected the
correct way round. Please take care with this because a wrongly connected
transistor may be damaged instantly when you switch on.

If you are lucky the orientation of the transistor will be clear from the PCB
or stripboard layout diagram, otherwise you will need to refer to a supplier's
catalogue to identify the leads.

Please note that transistor lead diagrams show the view from below with the leads towards you. This is the opposite of IC (chip) pin diagrams which
show the view from above.

Please see below for a table showing
the case styles of some common transistors.

Soldering

Transistors can be damaged by heat when soldering so if you
are not an expert it is wise to use a heat sink clipped to the lead between the
joint and the transistor body. A standard crocodile clip can be used as a heat
sink.

Do not confuse this temporary heat sink with the permanent heat sink
(described below) which may be required for a power transistor to prevent it
overheating during operation.

Heat sinks

Waste heat is produced in transistors due to the current
flowing through them. Heat sinks are needed for power transistors because they
pass large currents. If you find that a transistor is becoming too hot to touch
it certainly needs a heat sink! The heat sink helps to dissipate (remove) the
heat by transferring it to the surrounding air.

Testing a transistor

Transistors can be damaged by heat when soldering
or by misuse in a circuit. If you suspect that a transistor may be damaged there
are two easy ways to test it:

Testing an NPN transistor

1. Testing with a multimeter

Use a multimeter or a simple tester (battery, resistor and LED) to check each pair of leads for conduction. Set a
digital multimeter to diode test and an analogue multimeter to a low resistance
range.

Test each pair of leads both ways (six tests in total):

The base-emitter (BE) junction should behave like a diode and conduct one way only.

The base-collector (BC) junction should behave like a diode and conduct one way only.

The collector-emitter (CE) should not conduct
either way.

The diagram shows how the junctions behave in an
NPN transistor. The diodes are reversed in a PNP transistor but the same test
procedure can be used.

A simple switching circuit
to test an NPN
transistor

2. Testing in a simple switching circuit

Connect the transistor into the
circuit shown on the right which uses the transistor as a switch. The supply
voltage is not critical, anything between 5 and 12V is suitable. This circuit
can be quickly built on breadboard for example. Take
care to include the 10k resistor in the base connection or you will destroy the
transistor as you test it!

If the transistor is OK the LED should light when the switch is pressed and
not light when the switch is released.

To test a PNP transistor use the same circuit but reverse the LED and the
supply voltage.

Some multimeters have
a 'transistor test' function which provides a known base current and measures
the collector current so as to display the transistor's DC current gain
hFE.

Transistor codes

There are three main series of transistor codes used in
the UK:

Codes beginning with B (or A), for example BC108, BC478The first letter B is for silicon, A is for germanium (rarely used
now). The second letter indicates the type; for example C means low power
audio frequency; D means high power audio frequency; F means low power high
frequency. The rest of the code identifies the particular transistor. There is
no obvious logic to the numbering system. Sometimes a letter is added to the
end (eg BC108C) to identify a special version of the main type, for example a
higher current gain or a different case style. If a project specifies a higher
gain version (BC108C) it must be used, but if the general code is given
(BC108) any transistor with that code is suitable.

Codes beginning with TIP, for example TIP31ATIP refers
to the manufacturer: Texas Instruments Power transistor. The letter at the end
identifies versions with different voltage ratings.

Codes beginning with 2N, for example 2N3053The initial
'2N' identifies the part as a transistor and the rest of the code identifies
the particular transistor. There is no obvious logic to the numbering
system.

Choosing a transistor

Most projects will specify a particular
transistor, but if necessary you can usually substitute an equivalent transistor
from the wide range available. The most important properties to look for are the
maximum collector current IC and the current gain hFE. To
make selection easier most suppliers group their transistors in categories
determined either by their typical use or maximum power rating.

To make a final choice you will need to consult the tables of technical data
which are normally provided in catalogues. They contain a great deal of useful
information but they can be difficult to understand if you are not familiar with
the abbreviations used. The table below shows the most important technical data
for some popular transistors, tables in catalogues and reference books will
usually show additional information but this is unlikely to be useful unless you
are experienced. The quantities shown in the table are explained below.

NPN transistors

Code

Structure

Case
style

IC
max.

VCE
max.

hFE
min.

Ptot
max.

Category
(typical use)

Possible
substitutes

BC107

NPN

TO18

100mA

45V

110

300mW

Audio, low power

BC182 BC547

BC108

NPN

TO18

100mA

20V

110

300mW

General purpose, low power

BC108C BC183 BC548

BC108C

NPN

TO18

100mA

20V

420

600mW

General purpose, low power

BC109

NPN

TO18

200mA

20V

200

300mW

Audio (low noise), low power

BC184 BC549

BC182

NPN

TO92C

100mA

50V

100

350mW

General purpose, low power

BC107 BC182L

BC182L

NPN

TO92A

100mA

50V

100

350mW

General purpose, low power

BC107 BC182

BC547B

NPN

TO92C

100mA

45V

200

500mW

Audio, low power

BC107B

BC548B

NPN

TO92C

100mA

30V

220

500mW

General purpose, low power

BC108B

BC549B

NPN

TO92C

100mA

30V

240

625mW

Audio (low noise), low power

BC109

2N3053

NPN

TO39

700mA

40V

50

500mW

General purpose, low power

BFY51

BFY51

NPN

TO39

1A

30V

40

800mW

General purpose, medium power

BC639

BC639

NPN

TO92A

1A

80V

40

800mW

General purpose, medium power

BFY51

TIP29A

NPN

TO220

1A

60V

40

30W

General purpose, high power

TIP31A

NPN

TO220

3A

60V

10

40W

General purpose, high power

TIP31C TIP41A

TIP31C

NPN

TO220

3A

100V

10

40W

General purpose, high power

TIP31A TIP41A

TIP41A

NPN

TO220

6A

60V

15

65W

General purpose, high power

2N3055

NPN

TO3

15A

60V

20

117W

General purpose, high power

Please note: the data in this table was
compiled from several sources which are not entirely consistent! Most of
the discrepancies are minor, but please consult information from your
supplier if you require precise data.

PNP transistors

Code

Structure

Case
style

IC
max.

VCE
max.

hFE
min.

Ptot
max.

Category
(typical use)

Possible
substitutes

BC177

PNP

TO18

100mA

45V

125

300mW

Audio, low power

BC477

BC178

PNP

TO18

200mA

25V

120

600mW

General purpose, low power

BC478

BC179

PNP

TO18

200mA

20V

180

600mW

Audio (low noise), low power

BC477

PNP

TO18

150mA

80V

125

360mW

Audio, low power

BC177

BC478

PNP

TO18

150mA

40V

125

360mW

General purpose, low power

BC178

TIP32A

PNP

TO220

3A

60V

25

40W

General purpose, high power

TIP32C

TIP32C

PNP

TO220

3A

100V

10

40W

General purpose, high power

TIP32A

Please note: the data in this table was
compiled from several sources which are not entirely consistent! Most of
the discrepancies are minor, but please consult information from your
supplier if you require precise data.

Structure

This shows the type of transistor, NPN or PNP. The polarities of the
two types are different, so if you are looking for a substitute it must be
the same type.

Case style

There is a diagram showing the leads for some of the most common case
styles in the Connecting section above. This information is also available in suppliers'
catalogues.

IC max.

Maximum collector current.

VCE max.

Maximum voltage across the collector-emitter junction. You
can ignore this rating in low voltage circuits.

hFE

This is the current gain (strictly the DC current gain). The
guaranteed minimum value is given because the actual value varies from
transistor to transistor - even for those of the same type! Note that
current gain is just a number so it has no units. The gain is
often quoted at a particular collector current IC which is
usually in the middle of the transistor's range, for example '100@20mA'
means the gain is at least 100 at 20mA. Sometimes minimum and maximum
values are given. Since the gain is roughly constant for various currents
but it varies from transistor to transistor this detail is only really of
interest to experts. Why hFE? It is one of a whole
series of parameters for transistors, each with their own symbol. There
are too many to explain here.

Ptot max.

Maximum total power which can be developed in the transistor, note
that a heat sink will be required to achieve the maximum rating. This rating is important
for transistors operating as amplifiers, the power is roughly
IC × VCE. For transistors operating as switches the
maximum collector current (IC max.) is more important.

Category

This shows the typical use for the transistor, it is a good starting
point when looking for a substitute. Catalogues may have separate tables
for different categories.

Possible substitutes

These are transistors with similar electrical properties which will be
suitable substitutes in most circuits. However, they may have a different
case style so you will need to take care when placing them on the circuit
board.